Heretofore, there has been known a technique for protecting a semiconductor switching device (abbreviated as “device” below) in an inverter from thermal destruction (see Patent Literature 1).
In Patent Literature 1, an average temperature of a device is estimated from an average loss in a period of an output current. In a region where the motor revolution speed is high and the device temperature hardly pulsates during a period of the output current, the average temperature is used as an estimated temperature. On the other hand, in a region where the motor revolution speed is low and the device temperature largely pulsates during a period of the output current, the average temperature is multiplied by a coefficient based on the revolution speed, and a peak value of the pulsating temperature is used as an estimated temperature.